The invention is in the field of fluorescent lamp systems having one or more fluorescent lamps connected in electrical series with a ballast, for operation in the 200 to 300 volt range, such as standard values of 220, 240, or 277 volts.
Many fluorescent lamp systems have two lamps, usually positioned side-by-side, to provide more light from a larger source area than does a single lamp. Fluorescent lamps are manufactured in various different lengths, the 24-inch length and the 48-inch length being widely used in lighting systems. The voltage required for starting and operating fluorescent lamps increases approximately linearly with increased lamp length. Thus, a 48" lamp requires twice the voltage as a 24" lamp, two 24" lamps connected in series require essentially the same voltage as a single 48" lamp, and two 48" lamps in series require approximately twice the voltage as two 24" lamps in series.
A widely used type of fluorescent lamp circuit utilizes a voltage step-up transformer connected between the lamp(s) and the power line voltage supply, for providing suitable voltage to the lamp(s), such as is disclosed in U.S. Pat. No. 4,185,233 to Riesland et al. Another widely used fluorescent lamp circuit, which is more economical to manufacture, utilizes a current-limiting ballast impedance (such as an inductor, resistor, and/or capacitor) connected in series between the lamp(s) and the power line voltage source. The total length of the discharge path(s) of the lamp(s) in this circuit must be short enough so the lamp starting and operating voltages are sufficiently less than the line voltage. Therefore, such circuits have been limited to operating a single 24" (or shorter) fluorescent lamp from a 120-volt a-c line, and two 24" (or shorter) lamps connected in series (or a single 48" or shorter lamp) from a 220-volt, 240-volt, or 277-volt a-c line, the latter voltage being obtained from a 480-volt three-phase system. For convenience, these voltages are defined herein as being in a range of 200 to 300 volts.
It has been desirable to be able to have a lighting system employing a pair of 48-inch fluorescent lamps in series with a simple series ballast as shown in FIG. 1 for operation from a line voltage in the range of 200 to 300 volts, but this has not been achieved heretofore because of the higher voltage requirements of the 48-inch lamps as compared to the 24-inch lamps. Such a system, if it could be achieved, would provide about twice the light output of a 24-inch lamp system with a substantial increase in system efficacy at only a small increase in cost.